This invention relates to acrylic fibers for producing carbon fibers.
Carbon fibers are produced and used on a large scale as reinforcing fibers for composite materials to be used in many fields including aircraft, spacecraft, pressure vessels to be placed on the sea bed, and sporting goods such as golf shafts, tennis rackets, and fishing rods due to their excellent physical and chemical properties.
As the raw fiber materials for producing such carbon fibers, or precursors, viscose fibers, acrylic fibers, and pitch fibers are typically employed. It is well known that these precursors are converted to carbon fibers generally through the process of oxidizing them in an oxidative atmosphere at 200.degree. to 400.degree. C. to render them flame-resistant or infusible and carbonizing the thus oxidized fibers in an inert atmosphere at elevated temperatures of at least 800.degree. C.
The precursors to be rendered flame-resistant or infusible and then carbonized or graphitized under the above-described severe conditions can cause, in the heat treatment at elevated temperatures, particularly in the step of rendering the precursors flame-resistant or infusible, an adhering or sticking phenomenon (hereinafter referred to simply as adhering) between fibers and fluffing or breaking of fibers resulting from generation of mechanical defects of fiber surfaces. Thus, it is not necessarily easy to produce carbon fibers having definite quality and performance with good productivity.
That is, precursor fibers for producing carbon fibers, which are to be converted to oxidized fibers in the oxidation step of rendering them flame-resistant or infusible through complicated chemical reactions such as intermolecular crosslinking or intramolecular cyclization, suffer softening, partial adhering, and tar formation with the progress of the reactions in the above-described step, unavoidably leading to adhering between fibers and easy formation of fiber defects. The adhering between fibers and generation of fiber defects to be caused by the treatment of rendering the precursor fibers flame-resistant greatly depend upon the kind of oil composition deposited thereon. Oil compositions with a low heat resistant fail to prevent the adhering phenomenon and generation of fiber defects, and exert detrimental influences on the precursor fibers.
For removing the above-described troubles or problems with the production of carbon fibers, many proposals have been made on the composition of raw materials constituting precursor fibers (polymer composition, pitch composition, etc.) and on the treatment thereof with chemicals or oils. A proper oil composition for the precursor must be selected taking into consideration not only the troubles or problems encountered in the step of converting the precursor into carbon fibers but other factors as well. The oil composition to be deposited onto the precursor directly influences productivity, process stability, quality, performance, etc. of the precursor itself.
For example, silicone oils are known to be effective for preventing adhering between fibers in the aforesaid oxidation step for the production of carbon fibers using acrylic fibers as precursor fibers, and many silicone oils have been proposed, for example, in Japanese Patent Application (OPI) Nos. 103313/80 and 122021/80, and U.S. Pat. No. 4,259,307.
However, although these silicone oils reduce, to some extent, the adhering phenomenon between fibers in the oxidation step of converting them to oxidized fibers, acrylic fibers having been treated with the silicone oil are liable to generate static electricity, and fluffing, winding round rollers and guides, and breaking of fibers, etc. occurs thus rendering the process operation unstable.